(1) Field of the Invention
This invention relates to an exhaust tube for a vehicle for reducing vibration during idling time.
(2) Explanation of Related Art
As for a conventional exhaust tube for automobile, there is, for example, such one as shown in Nissan Service Weekly No. 428, issued Sept., 1980, Nissan Automobile Co., in which an exhaust tube provided for automobile has a bent portion of substantially L-shape at the mid portion and also a main muffler provided at the tail portion of the exhaust tube. In this kind of exhaust tube, at the region of the secondary frequencies of the revolving engine (around 20 to 30 Hz) during idling time, the exhaust tube acts like a spring and produces forward and backward resonance with the main muffler acting as a mass. This forward and backward resonance, especially in the case of FF vehicle with laterally disposed engine, causes large effect upon the vibration in the rolling direction of the engine, and deteriorates the vertical vibration of the steering wheels as well as the floor vibration. In order to prevent the deterioration of these vibrations, it is necessary to set the frequencies of the forward and backward resonance of the exhaust tube outside the region of the secondary frequencies of the revolving engine. When attempt is made to set the frequencies of this forward and backward resonance at a region of higher frequencies than the region of the secondary frequencies of the revolving engine, it is necessary to increase the rigidity of the exhaust tube.
In order to increase the rigidity of the exhaust tube, it is necessary to change the layout at the vehicle. Therefore, it becomes difficult to adopt this method. On the other hand, when attempt is made to set the frequencies of the forward and backward resonance at a region of lower frequencies than the region of the secondary frequencies of the revolving engine, it is only necessary to largely decrease the rigidity of the exhaust tube in the axial direction as well as the bent direction. For this purpose, there is adopted a method to replace a portion of the exhaust tube with a flexible tube.
However, in this kind of exhaust tube for vehicle, since the flexible tube is used for a portion of the exhaust tube, it becomes costly to manufacture, and also, since the diameter of the flexible tube is larger than the other exhaust tubes, there is a problem that the layout limitations increases.